1. Field of the Invention
The present invention relates to a method of monitoring the quality of acquired data. In particular, the present invention relates to a method of monitoring the quality of seismic data that can be carried out while seismic data is being acquired, so as to provide on-line quality control of the acquired data.
2. Description of the Related Art
It is highly desirable to provide quality control of data acquired during seismic exploration in real-time. By xe2x80x9creal-timexe2x80x9d is meant that the results of the quality control are produced at substantially the same rate that data is acquired; this is also known as xe2x80x9con-linexe2x80x9d quality control. Providing on-line quality control during a seismic survey enables any deterioration in quality of the acquired seismic data that might occur during the survey, for example as a result of the onset of a fault in a seismic source or a seismic receiver, to be detected and investigated while the survey is still in progress. In the absence of on-line quality control such a deterioration in the quality of the data would only become apparent once the data was analysed after the completion of the survey. In order to perform quality control on-line it is necessary to minimise the amount of computation required to perform the quality control, owing to the large amount of data that is acquired in a typical seismic survey.
A known method of providing on-line quality control of seismic data is based on determining the route-mean-square (RMS) amplitude of a seismic data trace. The RMS amplitude of a seismic data trace is a characteristic of the energy of the trace. Such a prior art on-line quality control method is illustrated schematically in FIG. 1. It can be seen that threshold levels are set for the normalised RMS amplitude of a seismic data trace, and the thresholds are used to classify a trace. In the example of FIG. 1, a trace having a normalised RMS amplitude between 0.0 and 0.1 is classified as a xe2x80x9cdeadxe2x80x9d trace, a trace having a normalised RMS amplitude in the range 0.1 to 0.3 is classified as a xe2x80x9cweakxe2x80x9d trace, and a trace having a RMS amplitude in the range 0.7 to 1.0 is classified as a xe2x80x9cnoisyxe2x80x9d trace. Acceptable traces have a RMS amplitude value in the range 0.3 to 0.7.
The RMS amplitudes of data traces are normalised using, for example, the expected amplitude of a data trace in a particular survey geometry having regard to the energy of the seismic source.
This prior art method of on-line quality control monitors only the RMS amplitude of the seismic data traces acquired in a seismic survey. The RMS amplitude of a seismic data trace is indicative of the energy of the seismic data, so that this prior art method monitors only the energy of the seismic data traces. Hence, the classification of the traces into xe2x80x9cdeadxe2x80x9d, xe2x80x9cweakxe2x80x9d, xe2x80x9cgoodxe2x80x9d and xe2x80x9cnoisyxe2x80x9d traces is based solely on the strength of the signal, and does not provide any information as to whether or not the signal is useful. Moreover, it is not possible for this prior art method to provide on-line monitoring of further characteristics of the seismic data, because the amount of computation that this would require could not be carried out at the same rate as the seismic data was acquired.
The present invention provides a method of monitoring the quality of data comprising the steps of: sampling a first data trace to determine the amplitude of the trace at a plurality of sampling times; and determining first and second attributes for the first data trace from the sampled amplitudes of the first data trace, the first and second attributes being indicative of the noise content of the first data trace.
The method of the present invention enables two or more attributes that relate to the noise content of the acquired data traces to be monitored on-line. This provides improved quality control compared with the prior art method that monitors just the energy of the data traces.
The method of the present invention involves sampling a data trace to determine the amplitude of the trace at intervals after the start time of the trace. (In the case of a seismic data trace, for example, this generally corresponds to the time at which the seismic source is actuated). Thus, the invention requires only time domain processing, and does not require frequency domain processing. This reduces the amount of computation required to determine the attributes of the data, and so allows the method of the invention to monitor more than one attribute of the data.
In a preferred embodiment, the method comprises the further step of comparing one of the first and second attributes determined for the first data trace with a predetermined threshold value. This provides information about the quality of the data trace relative to a pre-set threshold.
In an alternative embodiment, the method comprises the further steps of: sampling a second data trace to determine the amplitude at a plurality of sampling times; determining the first and second attributes for the second data trace from the sampled amplitudes of the second data trace; and comparing at least one of the first and second attributes determined for the first data trace with the corresponding one(s) of the first and second attributes determined for the second data trace. This embodiment provides a measure of the relative quality of the first and second data traces. If the first and second data traces are, for example, seismic data traces that were acquired in the same shot, this comparison provides information about the relative quality of traces within a shot; alternatively if the first and second data traces are, for example, seismic data traces that were acquired in different shots this comparison provides information about the relative quality of traces from different shots.
In a preferred embodiment, the or each data trace is a seismic data trace.
Preferred features of the present invention are set out in the dependent claims.